POTENTIAL OF TURBIDITY MONITORING FOR MEASURING THE TRANSPORT OF SUSPENDED-SOLIDS IN STREAMS

Investigating the transport of suspended solids by water sampling usua lly leads to an underestimation of loads and an unrealistically high s ampling frequency is required to properly characterize temporal trends . An alternative method is to use in situ optical turbidimeters to est imate the suspended solids concentration; however, the relationship be tween turbidity and suspended solids concentration is potentially conf ounded by variations in particle size, particle composition and water colour. Field measurements, and laboratory measurements using the type of natural material suspended in streamwater, were made to quantify t he influences of these factors on nephelometric turbidity (Hach 2100A) and attenuance turbidity (Partech 7000 3RP MKII). The attenuance turb idity was approximately 2.5 times higher than nephelometric turbidity. The turbidity instruments were most sensitive to dispersions with a m edian diameter of 1.2-1.4 mu m. Particle size variation can cause the turbidity to vary by a factor of four for the same concentration of su spended solids. However, the numerous close correlations between turbi dity and suspended solids concentration reported previously suggests t hat either the particle size variations are not usually great, or that particle size variations are often associated with variations in susp ended solids concentration. For the same concentration and particle si ze, organic particles gave attenuance turbidity values two to three ti mes higher than mineral particles. However, shortterm temporal variati ons from purely organic to purely mineral particle loads are rare in n ature, so variations in the percentage of organic matter in the partic ulate load will not confound turbidity to this extent. Coloured dissol ved organic matter is unlikely to alter the turbidity reading by more than 10%. An adequate relationship between turbidity measured in the f ield and suspended solids concentration should be expected in most sit uations. Some variance can be tolerated because a continuous estimate of suspended solids concentration overcomes the problem of infrequent sampling, which is the greatest source of error in the estimation of s tream sediment loads.